SEOUL (Reuters) -The national security advisers of the United States, South Korea and Japan are meeting in Seoul on Friday and Saturday to discuss North Korea and other global issues as they step up trilateral cooperation, South Korea’s presidential office said. Seoul’s national security adviser, Cho Tae-yong, held bilateral talks with Japan’s Takeo Akiba and the U.S.’ Jake Sullivan on Friday, during which they agreed to boost cooperation to curb North Korea’s nuclear threat and missile provocations, according to South Korea’s presidential office. They will hold a trilateral meeting on Saturda…